The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
Chemical cross-linkers are valuable tools for scientists and are discussed in numerous books and catalogues. See, e.g., Wong, Chemistry of Protein Conjugation and Cross-linking, CRC Press, Boca Raton, Fla., 1991. These reagents may be used in a variety of ways, such as to assist in the determination of near-neighbor relationships in proteins, molecular associations in cell membranes, three-dimensional structures of proteins, enzyme-substrate orientation, solid-phase immobilization, and hapten-carrier protein conjugation. They are also useful for preparing antibody-detectable label conjugates, immunotoxins and other labeled protein and nucleic acid reagents. Cross-linking agents often employ functional groups that couple to amino acid side chains of peptides. These reagents may be classified on the basis of the following:                1. Functional groups and chemical specificity;        2. length and composition of the cross-bridge;        3. whether the cross-linking groups are similar (homobifunctional) or different (heterobifunctional);        4. whether the groups react chemically or photochemically;        5. whether the reagent is cleavable; and        6. whether the reagent can be radiolabeled or tagged with another label.        
Reactive groups that can be targeted using a cross-linker include primary amines, sulfhydryls, carbonyls, carbohydrates and carboxylic acids. In addition, many reactive groups can be coupled nonselectively using a cross-linker such as photoreactive phenyl azides.
Cross-linking reagents contain at least two reactive groups, and are divided generally into homofunctional cross-linkers (containing identical reactive groups) and heterofunctional cross-linkers (containing non-identical reactive groups). While for convenience the following discussion refers to homobifunctional and heterobifunctional cross-linkers (where “bifunctional” refers to the presence of two functional groups), cross-linking reagents having more than two functional groups are well known to the artisan and are within the scope of the invention described herein.
Homobifunctional cross-linkers that couple through amines, sulfhydryls or react non-specifically are available from many commercial sources. Maleimides, alkyl and aryl halides, alpha-haloacyls and pyridyl disulfides are thiol reactive groups. Maleimides, alkyl and aryl halides, and alpha-haloacyls react with sulfhydryls to form thiol ether bonds, while pyridyl disulfides react with sulfhydryls to produce mixed disulfides. The pyridyl disulfide product is cleavable. Imidoesters are also very useful for protein-protein cross-links. These cross-linkers can penetrate cell membranes and cross-link proteins within the membrane to study membrane composition, structure and protein-protein and protein-lipid interactions. Imidoesters are also useful for oligomer formation. For example, cross-linking proteins to form oligomers may reveal if a bivalent, dimeric or trimeric form of the protein is responsible for activity.
A nonselective homobifunctional cross-linker is useful for conjugating functional groups, such as hydroxyls for which specific cross-linkers are not available. An example of a nonselective homobifunctional cross-linker is BASED (Product #21564 Pierce Co.). This cross-linker has a long spacer arm and 2 aromatic rings which makes it very hydrophobic with a limited solubility in aqueous systems. This cross-linker also has a large diffusion capacity and may be useful for permeation of biological membranes before conjugation initiates.
Heterobifunctional cross-linkers possess two or more different reactive groups that allow for sequential conjugations with specific groups of proteins, minimizing undesirable polymerization or self-conjugation. Heterobifunctional reagents are also used when modification of amines is problematic. Amines may sometimes be found at the active sites of macromolecules, and the modification of these may lead to the loss of activity. Other moieties such as sulfhydryls, carboxyls, phenols and carbohydrates may be more appropriate targets. A two-step strategy allows for the coupling of a protein that can tolerate the modification of its amines to a protein with other accessible groups. A variety of heterobifunctional cross-linkers, each combining different attributes for successful conjugation are commercially available. Cross-linkers that are amine-reactive at one end and sulfhydryl-reactive at the other end are quite common.
If using heterobifunctional reagents, the most labile group is typically reacted first to ensure effective cross-linking and avoid unwanted polymerization. A selection of heterobifunctional reagents that contain at least one photoaffinity group are also commercially available. This selection includes iodinatable and cleavable reagents that react nonspecifically at the azido group and with amines, sulfhydryls, carbohydrates and carbonyls.
Many factors must be considered to determine optimum cross-linker-to-target molar ratios. Depending on the application, the degree of conjugation is an important factor. For example, when preparing immunogen conjugates, a high degree of conjugation is normally desired to increase the immunogenicity of the antigen. However, when conjugating to an antibody or an enzyme, a low-to-moderate degree of conjugation may be optimal to ensure that the biological activity of the protein is retained. It is also important to consider the number of reactive groups on the surface of the protein. If there are numerous target groups, a lower cross-linker-to-protein ratio can be used. For a limited number of potential targets, a higher cross-linker-to-protein ratio may be required. This translates into more cross-linker per gram for a small molecular weight protein.
Conformational changes of proteins associated with a particular interaction may also be analyzed by performing cross-linking studies before and after the interaction. A comparison is made by using different arm-length cross-linkers and analyzing the success of conjugation. The use of cross-linkers with different reactive groups and/or spacer arms may be desirable when the conformation of the protein changes such that hindered amino acids become available for cross-linking.
Cross-linkers are available with varying lengths of spacer arms or bridges connecting the reactive ends. The most apparent attribute of the bridge is its ability to deal with steric considerations of the moieties to be linked. Because steric effects dictate the distance between potential reaction sites for cross-linking, different lengths of bridges may be considered for the interaction. Shorter spacer arms are often used in intramolecular cross-linking studies, while intermolecular cross-linking is favored with a cross-linker containing a longer spacer arm.
The inclusion of polymer portions (e.g., polyethylene glycol (“PEG”) homopolymers, polypropylene glycol homopolymers, other alkyl-polyethylene oxides, bis-polyethylene oxides and co-polymers or block co-polymers of poly(alkylene oxides)) in cross-linkers can, under certain circumstances be advantageous. See, e.g., U.S. Pat. Nos. 5,643,575, 5,672,662, 5,705,153, 5,730,990, 5,902,588, and 5,932,462; and Topchieva et al., Bioconjug. Chem. 6: 380–8, 1995). For example, U.S. Pat. No. 5,672,662 discloses bifunctional cross-linkers comprising a PEG polymer portion and a single ester linkage. Such molecules are said to provide a half-life of about 10 to 25 minutes in water.
Each reference cited in the preceding section is hereby incorporated by reference in its entirety, including all tables, figures, and claims.